It has been proposed that the nucleus, (like the cytoplasm and extracellular matrix), has a basic skeletal framework upon and around which nuclear structure and functions are organized. This framework, termed the nuclear matrix, is defined as the residual structure left after approximately 99% of nucleic acids and approximately 90% of proteins have been removed by successive treatments with nuclease, low salt, and high salt. The nuclear matrix hypothesis has been strengthened by the finding that many components associated with nuclear function (e.g., newly replicated DNA, HnRNA, steroid hormone receptors) are specifically associated with the matrix. However, the argument can also be made that the extraction processes needed to reveal the matrix actually generate it. Clearly, the validity of the hypothesis has great significance with respect to our understanding of the nuclear structure and function, as well as to the direction of research in this area. This proposal contains experiments designed to test the nuclear matrix hypothesis. We have developed a system in which inactive avian erythrocyte nuclei, lacking an internal nuclear matrix, can be induced to develop one following fusion with anucleate moust L-cell cytoplasts. During reactivation in the cytoplasts, the introduced nuclei also regain many nuclear functions including RNA synthesis, and chick-specific proteins appear in the cytoplasm. Firstly, we propose to identify the matrix polypeptides taken up by the erythrocyte nuclei and compare them with normal L-cell matrix polypeptides. Similar studies will be carried out using chick fibroblasts as a source of cytoplasts. By analogy with the cytoplasmic and extracellular skeletal proteins, the matrix hypothesis would predict that a common subset of polypeptides would be present in all cases. A failure to see major homologies between different types of matrix would suggest that the matrix, whatever its origin, is not a ubiquitous in vivo framework. Secondly, we will compare the development of the matrix with the development of RNA synthesis and transport. Proper functioning of the implanted nucleus should follow the development of the matrix rather than precede it.